The invention relates to an electromagnetic wave detector, and notably to a detector made of semiconductor materials with quantum well structures.
There are known quantum well based detectors of infra-red radiation using the transitions (a) between bound levels and free levels and (b) between bound levels. Their mode of operation is briefly recalled here below.
A detector of this type is formed by a stack of alternating layers of a small gap semiconductor (SGS) between two large gap semiconductors (LGS) as shown in figure 1a. The energy difference between the bottoms of the conduction bands of the two semiconductors is the "band-offset" .DELTA.E. For example, these two semiconductors may be made of GaAs for SGS and Al.sub.y Ga.sub.1-x As (where x is between 0 and 1) for LGS. The electrons in such a structure are subjected to a potential well with a depth .DELTA.E and a width d, where d is the width of the layer of SGS. If the width d is small enough, the energy of the electrons corresponding to the motion perpendicular to the layers is quantified in levels E.sub.1, E.sub.2 . . . In the bound/free photoconductive layers, the level E.sub.1 is bound (E.sub.1 &lt;.DELTA.E) and the level E is unbound or free (E.sub.2 &gt;.DELTA.E). If the level E.sub.1 is occupied because of an electron (by doping for example), a photon with energy hv greater than .DELTA.E-E.sub.1 causes an optical transition. The electron is then free to move and can be detected as a current at the terminals of the multiple well structure (see figure 1b).
The detector structure is shown then in FIG. 2. It has a stack of doped LGS/SGS/LGS layers sandwiched between two thick and highly doped SGS layers providing the ohmic contact. The device is at a temperature low enough for all the electrons to be trapped in the quantum wells. This current is given by the relationship: EQU J.sub.th =J.sub.tho e.spsp.-[(.DELTA.E-E.sbsp.1.spsp.)-E.sbsp.F.spsp.]/kT(1)
where
T is the temperature PA0 k is the Boltzmann constant, PA0 E.sub.F is the Fermi level of the electrons in the quantum well, PA0 J is the thermionic current given by K. Brennan and Y. Wang in: "Analysis of the Two-Dimensional Dark Currents in Quantum Well Devices", Applied Physics Letter, 57, 1337 (1990).
The response of this photodetector is deduced using the following line of reasoning. The density by volume of carriers is determined by the balance between, firstly, the optical generation: EQU G=.eta..multidot..phi./t
where .eta. is the quantum yield, .phi. the flux of the photons and t the thickness of the quantum well and, secondly, the recombination: EQU R=n/.tau.
where n is the density by volume of electrons in the conduction band of LGS and .tau. is the lifetime of this electron. The density of photo-excited electrons n is given by G=R, i.e. EQU n=.eta..tau..phi./t
and the current density by: EQU j=nq.mu.E EQU j=q.eta..tau..phi..mu.E/t (2)
where .mu. is the mobility of the electrons in the LGS and E is the applied electrical field. The response R of the structure is given by: EQU R=j/.phi.h.nu.=q.eta..iota..mu.E/.iota.h.nu.
It is observed that, for a constant electrical field, this response is independent of the number of wells. The invention relates to a device in which the response is a growing response when the number of wells increases. The response is then increased, and this increases the detectivity.